1. Field of the Invention
The present invention relates to thermal image transfer systems, to donor sheets useful in such systems, and to processes for thermally transferring images.
2. Background of the Art
Many imaging systems have been developed to be used with computer generated and other electronically generated images. This development has been necessitated by the generation or transmission of electronic images and the need for hard copy prints, both in black-and-white and color. Originally silver halide imaging systems were used for such imaging generation, and such systems still can provide high quality images. In certain areas of the market, lower image quality can be tolerated and lower costs are essential. Ink-jet printing and thermal dye transfer systems have found increasing acceptance in these markets.
Ink-jet printing has suffered in its acceptance because of a number of technical problems, not the least of which is a tendency of the print heads to clog. This requires an intolerable level of maintenance and a complete shut down of the system during servicing. Furthermore, image colors tended to be unstable and color gradation was virtually non-existent. Thermal colorant transfer systems have had fewer maintenance problems, but again image colors have not been stable where dyes are used as the colorant. Color gradation has also been quite limited in commercial systems, although significant improvements in these problems have been made in thermal colorant transfer systems.
The technology of thermal colorant transfer systems can generally be divided into two fields, mass transfer and dye sublimation transfer. The term mass transfer is used to refer to systems in which both the colorant and its binder are transferred from a donor sheet to a receptor sheet (or intermediate carrier sheet). Because of the relatively large size of the transferred material, a particle comprising both colorant and binder, color gradation or continuous tones in the image is difficult to achieve. Furthermore, if the colorant is a dye it exhibits more limited aging stability than do pigments.
The term dye sublimation transfer is used to refer to systems in which essentially only the colorant is transferred by sublimation or vaporization to a receptor sheet. This type of process would leave behind in the donor sheet any binder which might have been used in the donor sheet. This molecular transfer of colorant is capable of producing excellent continuous tone images because of the extremely small size of dye particles which can be transferred to the receptor sheet. There are two well defined problems with dye sublimation transfer systems, however. High energy levels (at least 6 Joules/cm.sup.2) are needed to transfer the dye. This results in both low output rates and excessive wear on the print head. Secondly, since the use of dyes is inherent in a sublimation or vaporization process, some image colors are unstable. To correct this problem, some dye sublimation transfer systems laminate a protective cover sheet to the color print image.
Various attempts have been made to eliminate or reduce the limitations described above. In the mass transfer area for example much of the improvement has occurred in the design and thermal control of the print head. A good example of this approch is given by S. Maruno of Matsushita Elec. Inc. Co., Ltd. in a paper presented at the August '86 SPSE Conference on Non-impact Printing Technologies in San Francisco. He described "thermo-convergent ink transfer printing" (TCIP) as a system in which the shape of the heating elements of the print head are optimized and the energy pulses to the head are controlled so that continuous tone reproduction is much improved when wax-colorant donor sheets are used.
Understandably the donor sheet itself has been the target of improvement work in recent years. Japanese Kokai No., J59224394 discloses the use of two incompatible binders in which the dye is dissolved. This results in the mass transfer of relatively small particles of colorant. Combining this donor sheet with good print-head control is reported to give some low level of color gradation.
European patent, EPO No. 163,297 teaches the use of high melting-point particles with diameters larger than the thickness of the ink layer which particles serve as heat conductors to aid in the transfer of the colorant mass.
A paper by Tagushi et al. of Matsushita given at the SPSE conference, August '86, in San Francisco briefly described a system claimed to yield improved mass transfer quality. This system makes use of one resin and colorant in the donor sheet and a different resin in the receptor sheet. The modulated thermal signal in the print-head causes changes in the "melt, compatibility, adhesion and transfer between the two resins" thus producing a continually graduated print.
In an effort to speed up the thermal mass transfer of wax/colorant systems, U.S. Pat. No. 4,541,043 describes an apparatus and method which makes possible the application of a solvent to the interface formed by the donor and receiving sheets.
Other examples of improved thermal mass transfer systems include: (a) donor sheets incorporating conductive/resistive layer pairs in their constructions as described in U.S. Pat. Nos. 4,470,714 and 4,588,315; and (b) donor sheets containing exothermic materials to amplify the energy provided by the print-head as taught in U.S. Pat. Nos. 4,491,432 and 4,549,824.
In the area of dye sublimation transfer, many attempts have been made to overcome the limitations of the system. The use of low molecular weight/melting point dyes has lowered the required transfer energy to some extent but this still remains higher than conventional commercial thermal print-head operating levels. Furthermore dyes of low melting-point tend to reduce image stability. Other changes in the area of dye sublimation include the use of binders in the receptor sheet which have an affinity for the vaporized or sublimed dye as taught in U.S. Pat. Nos. 4,490,435, 4,474,859 and 4,388,387.
EPO No. 011,004 discloses the use of a non-sublimable, crosslinked binder in which a sublimable dye is dispersed.
Although most or all of these attempts have been successful to some extent, none has given the desired combination of low transfer energy and full color, continuous tone images of excellent image color stability.